How do and should states respond to human security issues around the globe? And what does the answer to that question tell us about how international norms evolve to reÆ ect this activity? This chapter addresses the human security issue of mass atrocities and examines the evolution of international norms pertaining to the use of military force as a means to halt or avert such atrocities-what is widely known as humanitarian intervention. This is a topic that has received much attention in recent years, especially in light of the various interventions that occurred during the 1990s, the emergence of the Responsibility to Protect (R2P) doctrine, and, most recently, the intervention in Libya in March 2011 by NATO and allied forces.